HUMAN STRUCTURE AND LIFE- ACTIVITIES 475 



that it dips into water in a small tumbler or beaker. Or put some 

 of the paste in an osmose-apparatus.* After allowing an hour or 

 two for osmosis to take place, transfer some water from the tumbler 

 into the test-tube, and add some iodine solution. Compare with 

 test of starch as directed above. Is there evidence that starch 

 osmoses through a membrane? 



(D) Along with the above, test grape-sugar or corn-syrup by 

 placing in water inside a bag or in the tube of the osmose-apparatus. 

 Wait the same time as in the case of the starch, and test water in the 

 tumbler with Fehling's reagent ( 102). Does the sugar osmose ? 



(L) Mix some starch scraped from a potato in a drop of cold 

 water on a glass slide, put on cover-glass, and examine with low 

 power of microscope. Note the appearance of the starch-grains. 

 Now remove the slide from the microscope and heat slowly over an 

 alcohol- or a gas-burner until the water begins to steam. Examine 

 again with the microscope, and note the broken starch-grains. These 

 broken grains mixed with the water form starch paste, and the opales- 

 cent appearance of the thin paste is due to the numerous particles 

 of starch floating in the water. Examine starch scraped from a baked 

 or boiled potato. 



(D) Place some thin starch paste in each of two test-tubes. 

 Add to one tube (No. 1) a few drops of clear saliva (filtered through 

 coarse filter paper). Tube No. 2 has only paste. After twenty 

 to forty minutes note the appearance of the paste in the tubes, 

 especially with regard to opalescence. Pour a small quantity from 

 tube with the saliva (No. 1) into a clean test-tube, and add a few 

 drops of iodine solution. Is starch present ? 



(D) Into another tube pour some starch paste from tube No. 1, 

 and into still another pour paste that has been acted upon by saliva 

 in tube No 2. To each of the tubes apply Fehling's test. Re- 

 sults? Conclusions? Do these experiments suggest why a dry 

 starchy cracker becomes sweet to the taste after being held in the 

 mouth for some time? 



(D) If time permits, prepare three tubes with thin starch paste. 

 To No. 1 add boiled saliva ; to No. 2 add normal saliva, but add a 

 few drops of acid (vinegar will do) to make the paste slightly acid 

 (test with blue litmus-paper) ; and to No. 3 add saliva, but keep the 

 tube standing in a tumbler filled with finely cracked ice. After 

 twenty to forty minutes test for starch and sugar as in above. 

 What do these experiments show regarding (1) effect of boiling 



* Described in " Teachers' Manual.' 



